The present invention relates to a method for performing catalytic or non-catalytic processes, wherein oxygen is one of the reactants, comprising supply of oxygen from an oxygen containing gas mixture.
In conventional catalytic or non-catalytic processes where oxygen is one of the reactants, the supply of oxygen is usually performed by compressing air which then is treated in an air liquefaction or cryogenic unit. Recovered oxygen is usually compressed and to some extent reheated. In such processes the energy requirement related to the supply of oxygen comprises a substantial part of the total energy consumption of the process. Thus air liquefaction and compression and reheating of the oxygen stream will require large amounts of energy. Further, the equipments required for these process steps are expensive.
In the field of gas separation, several methods are known. A rather new technique is application of membranes made from materials with both ionic and electronic conductivity. Such a membrane can be a mixed oxygen ion and electron conducting membrane, for instance capable of separating oxygen from oxygen containing gaseous mixtures at 400-1300xc2x0 C. An oxygen partial pressure difference causes oxygen to be transported through the membrane by reduction of oxygen on the high oxygen partial pressure side (the feed side) and oxidation of the oxygen ions to oxygen gas on the low oxygen partial pressure side (the permeate side). In the bulk of the membrane, oxygen ions are transported by a diffusive process. Simultaneously the electrons flow from the permeate side back to the feed side of the membrane.
Application of mixed conducting membranes in separation of gas mixtures is generally known from the patent application EP 0 658 367 A2. This application shows separation of oxygen from air by means of a mixed conducting membrane which is integrated with a gas turbine system. Pure oxygen is recovered from the permeate side of the membrane unit at near atmospheric pressure or below. The oxygen thus has to be cooled to below approx. 50xc2x0 C. and recompressed to required process pressure before being supplied to the oxidation reactor.
The main objective of the invention was to arrive at improved methods for performing catalytic and non-catalytic processes in which oxygen is one of the reactants.
Another objective was to supply oxygen to said processes in a way that implied reduced energy consumption and investment costs.
A further objective was to utilise existing process streams in obtaining a cheaper oxygen supply to the process.
It was also an objective to apply the basic concept for production of nitric acid.
One problem the inventors faced in their search for a cheaper oxygen supply, was that the basic processes should not be substantially changed. Further, it would be an advantage to be able to utilise existing hot process streams. The inventors therefore started to look for solutions that might meet both these requirements.
The fact that catalytic or non-catalytic processes where oxygen is one of the reactants usually comprise at least one process stream being at elevated temperature would meet one of the requirements for utilising mixed conductive membranes. However, in order to obtain a high separation efficiency and to avoid cooling and recompression of oxygen an applicable sweep gas for the permeate side of the membrane should be available. One requirement was that the application of a sweep gas in said processes should not require supply of any additional reactant or gases and that it should be possible to make the sweep gas without installation of expensive new process equipment.
After having evaluated various ways of supplying oxygen to said catalytic or non-catalytic processes, the inventors decided to further investigate the possibility of applying mixed conductive membranes in spite of the teachings of the above EP-application. It was then found that in the processes in question there would be available various gas streams at elevated temperature making them applicable as sweep gas. Such sweep gas could be other reactants, steam, the products from a catalytic or non-catalytic chemical reaction, or recycled inert gases. Hot compressed air could be fed to a mixed conducting membrane which on the permeate side was exposed to a sweep gas of the above stated type and then picking up oxygen from the membrane unit. The sweep gas thus enriched with oxygen could then be transferred to a reactor for the final production of the desired products.
Application of a sweep gas in combination with a solid electrolyte membrane to lower the oxygen partial pressure to increase the degree of oxygen removal or oxygen recovery is known from the U.S. Pat. No. 5,035,726. In this patent a method for purifying crude argon is disclosed by selective permeation of oxygen through the membrane. Crude argon is compressed and heated and fed to a membrane unit to produce an O2 depleted argon stream. In order to improve the efficiency of gas separation by the membrane, the permeate side of the membrane is swept by available nitrogen supplied from a cryogenic unit.
In the said argon purification process the use of a high temperature heat exchanger is required to produce a sweep gas having a temperature above at least 500xc2x0 C. and preferably above 600 to 700xc2x0 C. to not cool the membrane and thus reduce the rate of oxygen transport through the membrane. In rather small process plants, or in the case of low oxygen flows preheating of the sweep gas in a high temperature heat exchanger could be economically feasible, but for recovery of several ton of oxygen per day the use of high temperature heat exchangers to preheat the sweep gas could be very expensive and probably not economical. By applying an available hot process stream as sweep gas the problems associated with the said heating of the sweep gas in the above U.S. patent is avoided.
Application of pure oxygen as an oxidant in the ammonia oxidation process is generally known from U.S. Pat. No. 3,927,182. According to this patent oxygen is supplied from an oxygen plant, e.g. a cryogenic plant, and said oxygen is mixed with recycled tail gas and the thus formed oxygen containing tail gas is compressed in a recycle compressor and is then mixed with evaporated ammonia and the formed gas mixture is fed to an ammonia burner. The main disadvantage of this process is the high energy requirement and the high cost of oxygen.
The present invention will thus in its widest scope comprise a method for performing a catalytic or non-catalytic process wherein oxygen is one of the reactants being supplied from an oxygen containing gas mixture, and where oxygen is picked up from the permeate side of an oxygen ion and electron conducting membrane by means of a sweep gas at elevated temperature and where the thus formed sweep gas is a reactant for the catalytic or non-catalytic oxidation process downstream the membrane unit from here defined as the main reactor.
In said method the sweep gas can be preheated in a catalytic or non-catalytic reactor located upstream the membrane unit by burning of fuel from here defined as a sweep gas preheater.
A further embodiment of the invention comprises that the process gas used as sweep gas is the gas phase product or part of the gas phase product from the main reactor.
Another embodiment of the invention comprises that process gas containing oxygen not utilised in the main reactor is applied as oxidant in the sweep gas preheater.
The process gas used as sweep gas can be steam containing a gas mixture produced by mixing the steam with about stoichiometric amounts of fuel and air. The thus formed mixture can be fed to a sweep gas preheater to increase the temperature of the steam containing sweep gas to between 400 and 1300xc2x0 C.
The composition and oxygen content of the sweep gas leaving the membrane unit can be regulated to give the desired temperature rise and gas composition of the product leaving the main reactor by regulation of the feed streams to the sweep gas preheater and the amount of oxygen transferred to the permeate side of said membrane unit.
The temperature in the sweep gas preheater can be controlled by addition of a coolant such as steam, water, inert gas or gas mixtures not taking part in the reaction in the main reactor.
The sweep gas having picked up oxygen from the membrane unit can be used in production of syngas for ammonia production by being reacted with a hydrocarbon fuel.
The process gas used as sweep gas is the NO-containing gas leaving the ammonia burner in a nitric acid process and the thus formed oxygen containing gas can be used in the production of nitric acid.
The main product is formed in the sweep gas preheater, and the gas leaving the sweep gas preheater is applied as sweep gas in the membrane unit. The sweep gas containing oxygen is cooled and separated in one product stream and one stream containing oxygen is recycled to the sweep gas preheater as oxidant in said preheater.